GCSEBiologyWJECExam Questions2. Variation, Homeostasis & Micro-organisms2.7 Micro-organisms & their Applications

Micro-organisms & their Applications (WJEC GCSE Biology): Exam Questions

Exam code: 3400

31 mins3 questions
1a2 marks

Mair investigated four antibiotics (A, B, C and D) to compare how effective they were at killing bacteria.

  • She set up four agar plates with bacteria growing on them.

  • She placed a filter paper disc containing a different antibiotic in the centre of each plate.

  • She put the plates in an incubator.

  • After three days, she measured the diameter of the clear zone where bacteria had been killed.

  • She calculated the area of the clear zone.

Image 6.1

Two diagrams of agar plates: one newly prepared with bacteria and an antibiotic disc, the other after three days showing a clear zone around the disc.

Mair used aseptic techniques to set up the agar plates.

(i) Explain why it was necessary for her to use sterilised forceps to place the filter paper discs on the agar.

[1]

(ii) Describe how she would have made sure that no bacteria from the environment could enter the agar plates after the filter paper discs were placed on the agar.

[1]

How did you do?

1b1 mark

Circle the most suitable temperature for the incubator in a school laboratory from the choice below.

10°C 25°C 37°C 60°C

How did you do?

1c9 marks

Table 6.2 shows Mair’s results.

Table 6.2

Antbiotic A

Antibiotic B

Antibiotic C

Antibiotic D

Area of clear zone on agar plate (mm2)

270

360

...............

450

(i) The radius of the clear zone for antibiotic C was 11 mm.

Complete Table 6.2 by calculating the area of the clear zone for antibiotic C, using the formula below, where r = radius.

Area of circle = straight pi r squared (straight pi= 3.14)

Give your answer to the nearest whole number.

[3]

(ii) Use the data in Table 6.2 to complete Bar chart 6.3 by:

[3]

I. adding the scale for the area of the clear zone .
II. drawing bars for antibiotics A, B and C and labelling your bars.

Bar chart showing the area of a clear zone in mm² for antibiotic D, with a single tall blue bar representing the data on a grid background.

(iii) From Bar chart 6.3, state which one of the antibiotics was the most effective and explain the reason for your choice.

[3]

Antibiotic ..............................
Reason ......................................

How did you do?

1d2 marks

(i) State how Mair could check that her results were reproducible.

[1]

(ii) Suggest one way in which the investigation could be extended to give more information about the effects of antibiotics on the growth of bacteria.

[1]

How did you do?

2a3 marks

A class of students compared the effects of two antibiotics and an extract from the seeds of moringa plants on the growth of bacteria.
Each group of students was given two agar plates. One plate had bacterium B.cereus growing on it whilst the other had the bacterium E.coli growing on it.
The students used aseptic techniques to place three filter paper discs labelled A, B and C on each plate. Each disc had each been soaked in one of the following solutions as shown in Table 6.1.

Table 6.1

Filter paper disc

Solution

A

moringa seed extract

B

penicillin (an antibiotic)

C

tetracycline (an antibiotic)

The plates were incubated for 24 hours. The results are shown in Image 6.2.

Image 6.2

Diagram of two agar plates. Left: B. cereus with discs A, B, C. Right: E. coli with discs A, B, C; disc C shows a clear zone indicating inhibition.

The students measured the diameter of the clear zones and recorded them in Table 6.3.

Table 6.3

Bacterium

Diameter of clear zone (mm)

Disc A
moringa seed extract

Disc B
penicillin

Disc C

tetracycline

B.cereus

11

8

12

E.coli

9

8

18

(i) State two steps the students would have used to maintain aseptic conditions whilst placing the discs on each plate.

[2]

(ii) State a suitable temperature the students would have used to incubate the agar plates in a school laboratory.

[1]

How did you do?

2b4 marks

(i) Explain how the clear zones formed on the agar plates shown in Image 6.2.

[2]

(ii) State two conclusions which can be made about the effect of the moringa seed extract on the growth of bacteria compared to the two antibiotics.

[2]

How did you do?

2c1 mark

Suggest one way in which each group of students in the class could have increased the validity of their conclusions.

How did you do?

3a5 marks

Environmental health officers estimated the number of bacteria in a milk sample from a cafe. The limit for the total number of bacteria in a sample of milk which is considered to be safe for human consumption is 1.0 × 105 bacteria per cm3 .

  • Using aseptic techniques, they diluted the sample by a factor of 10000.

  • They plated 1cm3 of the diluted sample onto nutrient agar.

  • The lid of the plate was secured with tape.

  • The plate was incubated at 37°C f for 2 days

  • The result is shown in Image 6.1.

Image 6.1

Petri dish with nutrient agar, labelled, showing multiple white bacterial colonies of varying sizes across its surface.

(i) State the assumption that must be made when calculating the number of bacterial cells present in the original sample.

[1]

(ii) I. Calculate the number of bacteria in 1cm3 of the original sample taken by the environmental health officers. Write your answer in standard form.

[3]

Number of bacteria = ...............................................

II. Conclude whether the milk sample was safe for human consumption. Explain your answer.

[1]

How did you do?

3b1 mark

Suggest why the plates were incubated at 37°C.

How did you do?

3c3 marks

(i) State why the lids of the agar plates were secured with tape.

[1]

(ii) State two other precautions that should have been taken to ensure aseptic technique.

[2]

How did you do?